clone_repo script does not fail if the branch does not exist, but
checks out the master branch [1], so we have to make sure the required
branch exists.
This patch also adds a check that there were no more patches merged
after the given branch was tagged EOL.
[1] https://opendev.org/openstack/releases/src/branch/master/tools/clone_repo.sh#L187
Change-Id: Ia1ededfc3cebbae807719750dc5e2d5949ce9a21